QUASI-COHERENT STRUCTURES IN THE MARINE ATMOSPHERIC SURFACE-LAYER

This paper presents a study aimed at identifying burst structures in the marine atmospheric surface layer. Standard ejection detection schemes, including the the quadrant, the variable interval time averaging, and the modified u-level techniques, were applied to turbulent wind data measured over the ocean. It was found that the proportion of contribution to the Reynolds stress from the four quadrants of the u'w' plane is in close agreement with the corresponding contributions in laboratory flows. Ejection detection by the quadrant technique followed by the grouping of ejections into bursts yielded a mean burst period of 47 s at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the short-time autocorrelation of the streamwise velocity signal and the first moment of the stress cospectrum. Phase averages of these events show a structure which is similar to the structure of the events detected in laboratory flows. These events do not scale on the ''inner'' variables, friction velocity u* and the kinematic viscosity v, as has been suggested for laboratory flows. Both ejection and burst periods decrease with increasing wind speed, however, the grouping of ejections into bursts breaks down at higher wind speeds. These periods showed no dependence on the surface wave field.